Toxic halogenated compounds significantly contribute to the problem of disposing of chemical wastes. Such wastes are either shipped hundreds of miles from shore to be incinerated or are stored in dumps for toxic wastes. In the past, negligence in locating such dumps has had catastrophic consequences in exposing large populations to the adverse health affects of toxic compounds. As a result, massive clean-up efforts are being undertaken to degrade and detoxify these compounds.
There is thus a clear need for an efficient and safe method to degrade toxic halogenated waste to harmless and environmentally compatible products. However, such a method has been elusive because of certain basic considerations with respect to the nature of toxic halogenated compounds. One aspect of the problem relates to the wide diversity of such compounds, ranging from such simple molecules as carbon tetrachloride and chloroform to complex insecticides such as p-p'-dichlorodiphenyltrichloroethane (p-p'-DDT). Another aspect of the problem relates to the theoretical consideration that as one increases the number of halogen atoms covalently joined to a tetrahedral carbon atom, one should experience a striking decrease in reactivity of the halogen atoms as a result of increasing steric hindrance to inversion of the tetrahedral configuration. This is consistent with a mechanism involving bimolecular nucleophilic substitution (S.sub.N 2); see San Fillipo, J., Jr.; Chern, C.-I.; Valentine, J. S.; "The Reaction of Superoxide with Alkyl Halides and Tosylates", J. Org. Chem., 1975, 40, 1678, and Johnson, R. A.; Nidy, E. G.; "Superoxide Chemistry. A Convenient Synthesis of Dialkyl Peroxides", J. Org. Chem., 1975, 50, 1680. Indeed, the value of the second order rate constant for the reaction of KO.sub.2 with methyl bromide (670.+-.20M.sup.-1 s.sup.-1) is more than 50 times greater than a similar reaction with methylene bromide (12.8.+-.0.2M.sup.-1 s.sup.-1), as reported by Danen, W. C.; Warner, R. J.; Arudi, R. L. in "Nucleophilic Reactions of Superoxide Anion Radical", Organic Free Radicals; ACS Symposium Series 69; Pryor, W. A., ed.; 1978; pp. 244-257. Our own experiments show that methyl chloride (80.+-.10M.sup.-1 s.sup.-1) reacts with superoxide ion in an aprotic solvent about nine times faster than methylene chloride (9.+-.12M.sup.-1 s.sup.-1).
In the Danen et al article the decrease in reactivity upon halogen substitution in the alpha position is attributed, at least in part, to steric hindrance, and closely related thereto, the effect called neighboring orbital overlap, attributable to electron repulsion between the incoming nucleophile and the alpha position halogen. These findings also are consistent with experiments reported in the following literature references:
Sawyer, D. T.; Gibian, M. J.; "The Chemistry of Superoxide Ion", Tetrahedron, 1979, 35, 1471. PA1 Merritt, M. V.; Sawyer, D. T.; "Electrochemical Studies of the Reactivity of Superoxide Ion with Several Alkyl Halides in Dimethyl Sulfoxide", J. Org. Chem, 1980, 35, 2157. PA1 Dietz, R.; Forni, A. E. G.; Larcombe, B. E.; Peover, M. E.; "Nucleophilic Reactions of Electrogenerated Superoxide Ion", J. Chem. Soc., B, 1980, 816. PA1 San Fillipo, J., Jr.; Romano, L. J.; Chern, C.-I.; Valentine, J. S., "Cleavage of Esters by Superoxide", J. Org. Chem., 1976, 41, 586. PA1 Magno, F.; Bontempelli, G.; "On the Reaction Kinetics of Electrogenerated Superoxide Ion with Aryl Benzoates", J. Electroanal. Chem., 1976, 68, 337 PA1 Gibian, J. J.; Sawyer, D. T.; Ungermann, T.; Tangpoonpholvivat, R.; Morrison, M. M., "Reactivity of Superoxide Ion with Carbonyl Compounds in Aprotic Solvents", J. Am. Chem. Soc., 1979, 101, 640. PA1 Magno, F.; Seeber, R.; Valcher, S., "A Study of the Reaction Kinetics of Electrogenerated Superoxide Ion with Benzylbromide", J. Electroanal. Chem., 1977, 83, 131.
Halogenated toxic wastes, of course include polyhalogenated hydrocarbons, and particularly pernicious are those compounds containing at least three halogen atoms covalently joined to a tetrahedral carbon atom. Accordingly, the diverse nature of the toxic waste mixture and the theoretically based limitation on reactivity of multi-halogen carbon atoms accentuate the formidable nature of the problem.
In accordance with the present invention, a process is provided which overcomes the foregoing problems and in particular is one which appears to fly in the face of theoretical limitations. Specifically, and surprisingly, we have discovered that when a compound has at least three halogen atoms covalently joined to a tetrahedral carbon atom, it reacts rapidly and efficiently, provided that the reactant is superoxide ion or hydroxide ion in an aprotic solvent. For example, we have discovered that the reaction of carbon tetrachloride with superoxide ion in an aprotic solvent proceeds at a reaction rate that is about 140 times faster than the reaction rate of methylene chloride with superoxide, and that chloroform is about 50 times faster in reaction rate than methylene chloride. While not desiring to be limited by any particular theory, it can be postulated that the inductive effect of the additional alpha halogen atoms unexpectedly overcomes the limiting effect of steric hindrance of the polyhalogenated carbon atom.
We have also found that an efficient method for converting toxic halides is to conduct the foregoing reaction in a controlled-potential electrolysis cell, wherein the superoxide ion is electrolytically generated in an aprotic solvent electrolyte. While it is known to electrolytically generate superoxide ion for reaction with alkyl halide in an aprotic solvent such as dimethyl sulfoxide (e.g. Merritt et al, supra), because of the constraints outlined above, such a process has not heretofore been used for the degradation or detoxification of polyhalogenated waste of the type effectively treated in accordance with the present invention.
More particularly, we provide a process for the degradation of a halogenated carbon compound capable of undergoing bimolecular nucleophilic substitution and containing at least three halogen atoms, comprising reacting the carbon compound in an aprotic solvent with a strong nucleophile selected from superoxide ion and hydroxide ion. The process is particularly suitable where at least three of the halogen atoms are covalently joined to a tetrahedral carbon atom. In a more particular embodiment, the nucleophile is electrolytically generated in an electrolytic cell fitted with an anode and a cathode and containing the aprotic solvent and an electrolyte adjacent the cathode. The halogenated carbon compound as such or dissolved in hydrocarbon solvent is introduced into the aprotic solvent in the cathode.
In a still further embodiment, specifically applicable to nucleophilic reaction with carbon tetrahalide, the aprotic solvent is dimethyl sulfoxide, the reaction forming dimethylsulfone and a carbonate which precipitates and can be separated from the reaction solution. While peroxides are known to oxygenate dimethyl sulfoxide to dimethylsulfone (Goolsby, A. D.; Saywer, D. T.; "The Electrochemical Reduction of Superoxide Ion and Oxidation of Hydroxide Ion in Dimethyl Sulfoxide", Anal. Chem., 1968, 40, 83), reaction with the peroxide degradation product of carbon tetrahalide is believed unique.